As a system for compressing the bandwidth of a television signal, a multiple sub-sampling encoding/decoding system is known. MUSE (Multiple Sub-Nyquist-Sampling Encoding) is one of such multiple sub-sampling sampling encoding/decoding systems which can effectively compress and transmit the broad bandwidth television signal.
The outline of the bandwidth compression technique by MUSE is as follows: first, the broad bandwidth television signal is A/D (analog-to-digital) converted at a sampling frequency of 48.6 MHz; second, the resultant digital signal undergoes the interfield offset sub-sampling; third, the interfield offset sub-sampled signal undergoes the interframe offset sub-sampling to form a transmission signal after the interfield offset sub-sampled signal is filtered by a low-pass filter, the cutoff frequency of which is lower than the sampling frequency of the interframe offset sub-sampling. In this processing, it is preferable that the interfield offset sub-sampling frequency is set at 24.3 MHz, the interframe offset sub-sampling frequency is set at 16.2 MHz, and the cutoff frequency is set at 12.15 MHz, the ratio of the interfield to interframe offset sub-sampling frequency being 3:2. A MUSE encoder and a MUSE decoder, which are used in the present invention and will be described later, are preferable examples that satisfy the above requirements.
Details of the MUSE system are described in U.S. Pat. No. 4,745,459, and U.S. Pat. No. 4,692,801, and in "Concept of the MUSE System And Its Protocol" in NHK Laboratories Note Ser. No. 348 published by the assignee of the present invention, which are incorporated herein by reference.
This system, i.e., the 1125/60 high definition television system proposed by NHK (Nippon Hoso Kyokai or Japan Broadcasting Corporation), is characterized by having 1125 scanning lines, an aspect ratio of 16:9, and 2:1 interlace scanning, with a field frequency of 60 Hz. The luminance signal bandwidth is about 20 MHz, while the chrominance signal bandwidth is approximately 7.0 MHz.
A system for the transmission of such a high definition color television signal is disclosed in U.S. Pat. No. 4,745,459 issued May 17, 1988, incorporated herein by reference. This system provides bandwidth reduction of the high definition video signal from approximately 26 MHz to 8.1 MHz. However, such a system cannot be used for terrestrial broadcast over single conventional transmission channels which are limited to 6 MHz in bandwidth. Even further, the high definition television signal of the type referred to is a time compressed and time division multiplexed signal in which luminance and chrominance signals are multiplexed in time to eliminate interference between the chrominance and luminance signals. Therefore, even if such a signal required a bandwidth less than 6 MHz, it could not be decoded and displayed on a conventional receiver such as an NTSC receiver, since the NTSC television signal is a composite signal in which a luminance signal is frequency multiplexed with a color subcarrier which is itself modulated by a chrominance signal and the two signal formats are thus not compatible with each other.
In referring to high definition television systems proposed for use in the United States and where compatibility with the existing NTSC system is the object, it has become a practice to designate such systems as "channel compatible" and/or "receiver compatible". A channel compatible system is said to be one in which the high definition signal can be encoded and broadcast transmitted with the presently assigned 6 MHz bandwidth frequency channels of the NTSC system. A receiver compatible system is one in which the signal can be decoded by and displayed by a conventional NTSC television receiver.
The baseband width of the MUSE system is set at 8 MHz because the MUSE system was originally developed for the broad bandwidth television broadcasting called "Hi-vision" which utilizes a satellite broadcast. The video signal is subjected to the bandwidth compression so as to be transmitted within a band of 8 MHz, and the audio signal is multiplexed into the vertical retrace interval using the Near-instantaneous compressing type DPCM (Differential Pulse-Code Modulation). Consequently, the broad bandwidth television signal must be further compressed to be transmitted by an earth station, for example, by the NTSC system using 6 MHz transmission bandwidth. Furthermore, a method for compensating the transmission characteristics must be considered.
As a bandwidth compression method, i.e., a way to reduce the amount of information to be transmitted, the following are possible: the reduction in the number of transmitted pixels in the horizontal direction; and/or the reduction in the number of scanning lines in the vertical direction. Considering the advantage of using, as a basic bandwidth compression method, an algorithm similar to the MUSE system that has already been developed as a bandwidth compression algorithm, reduction in the sampling frequency of the video signal from 48.6 MHz of the MUSE system to 43. 74 MHz, together with reduction in the number of scanning lines in the vertical direction from 1125 to 750, for example, makes it possible for the transmission bandwidth to be reduced to less than 6 MHz which is compatible with the NTSC system. The sampling frequency of 3.74 MHz and the number of scanning lines of 750 are not the essential restrictions. However, they are considered to be reasonable values because the ratio 1125:750 reduces to a simple integer ratio 3:2, which approximately corresponds to the Kell factor.
Once the sampling frequency and the number of scanning lines have been specified, a novel encoder and decoder relating to this method can be designed by using the MUSE technique already developed. However, the sampling frequency and the number of scanning lines are different from those of the MUSE system, and hence, the interfield offset sampling frequency, interframe offset sampling frequency, and the characteristics of prefilters handling those sampling frequencies are different from those of the MUSE encoder and decoder. Consequently, entirely new design and fabrication of a system including a scanning line number converter and a scanning line number reverse converter are required, which necessitates great amount of cost and labor because of its complicated arrangement as clearly seen from the block diagrams of MUSE encoders and MUSE decoders described in the documents mentioned above. In particular, considering the fact that practical apparatus are constructed by using integrated circuits, designing and fabrication require a great amount of cost and labor.